Coupler Insertion Loss
Application Note
Equation for the loss of the main line due to power loss through coupling factor. This does not account for material losses. The table at the bottom of this page has pre-calculated loss for most coupling values.
Recall dB = 10∙log(Pout/Pin).
- Consider a 6 dB coupler.
- If a 1 watt signal is applied to the INPUT, then 0.25 watts appears at the COUPLED port.
- This comes from -6dB = 10∙log(1/4).
- Consequently, 0.75 watts must therefore be present at the OUTPUT port, in a lossless coupler, because total energy out = total energy in.
- In this example, the measured loss from INPUT to COUPLED is -6dB = 10∙log(1/4).
- The measured loss from INPUT to OUTPUT is therefore -1.25dB = 10∙log(3/4).


Further Discussion
Please remember, this is a mathematical model based on an ideal situation. This demonstration does not factor in additional loss due to material properties and imperfect matching.
To understand the maximum specified insertion loss for a particular coupler model, always consult the datasheet.
Coupling (dB) | Main Line Insertion Loss (dB) |
---|---|
3.0 | 3.021 |
3.5 | 2.570 |
4.0 | 2.205 |
4.5 | 1.903 |
5.0 | 1.651 |
5.5 | 1.438 |
6.0 | 1.256 |
6.5 | 1.101 |
7.0 | 0.967 |
7.5 | 0.850 |
8.0 | 0.749 |
8.5 | 0.661 |
9.0 | 0.584 |
9.5 | 0.517 |
10.0 | 0.458 |
11.0 | 0.359 |
12.0 | 0.283 |
13.0 | 0.223 |
14.0 | 0.176 |
15.0 | 0.140 |
16.0 | 0.110 |
17.0 | 0.088 |
18.0 | 0.069 |
19.0 | 0.055 |
20.0 | 0.044 |
21.0 | 0.035 |
22.0 | 0.027 |
23.0 | 0.022 |
24.0 | 0.017 |
25.0 | 0.014 |
26.0 | 0.011 |
27.0 | 0.009 |
28.0 | 0.007 |
29.0 | 0.005 |
30.0 | 0.004 |
35.0 | 0.001 |
40.0 | 0.000 |
45.0 | 0.000 |
50.0 | 0.000 |
- Provided for convenience only. Not responsible for errors. Use at your risk.